In the development of brake boosters it became evident that by dividing the interior cavity of a housing by two diaphragm members a resultant output force could be significantly increased. In some of the resulting tandem brake boosters such as disclosed U.S. Pat. No. 5,233,911 communication between the chambers is achieved through passages in a hub that retains a control valve that is responsive to an operator input for the development of a pressure differential across the diaphragms that separate a rear chambers from a front chambers while in other tandem brake boosters such as disclosed in U.S. Pat. Nos. 3,083,698; 3,517,588 and 3,760,693 the rear chambers are connected to each other through flow path created between a peripheral surface of a partition member and housing. A partition member that is usually made of a plastic material engages the hub to separates a front chamber from an adjacent rear chamber. In some of such boosters, the partition member may move or float within the interior cavity as a function of a pressure differential developed during a brake application. In any event, the hub moves from a position of rest within the housing to an actuation position that is in a direct relationship to the movement of the plunger by the input force. Thus, when an operator applies an input force on the brake pedal, the arcuate travel of the brake pedal is translated into linear movement of the plunger. Unfortunately a desired brake force may not always be achieved if a total linear travel of the plunger occurs prior to the ultimate arcute travel of the brake pedal as manual input is not added to the output force when a maximum boost assist is reached.